1. Field of the Invention
The present invention relates to a high pressure fuel injection system suitable for an internal combustion engine of a type where fuel, such as, gasoline is highly pressurized and injected directly into engine cylinders.
2. Description of the Prior Art
Japanese First (unexamined) Patent Publication No. 64-36977 discloses a fuel injection system for an internal combustion engine. In this publication, the system includes a pressure regulating valve for adjusting a pressure of fuel supplied to fuel injection valves depending on a boost pressure (negative pressure) in an intake manifold of the engine, and further includes a mechanism for preventing leakage of the fuel out of the fuel injection valves when the engine is stopped. Specifically, the disclosed system relates to a low pressure fuel injection system for the engine of a type where the fuel is pressurized to a relatively low pressure, such as, about 0.2 MPa to 0.3 MPa and injected into the intake manifold. After the engine is stopped, the boost pressure reaches an atmospheric pressure and thus a pressure in a-control pressure chamber in the pressure regulating valve which introduces the boost pressure also becomes the atmospheric pressure. On the other hand, a fuel pump continues to rotate due to inertia for a short while after the engine is stopped. This results in increase of a discharge pressure of the fuel pump to the maximum extent, and thus further results in leakage of the highly pressurized fuel out of the fuel injection valves to wet inner walls of the intake manifold. This causes the overly-enriched fuel to deteriorate the start-up operation of the engine, particularly under a condition where the fast idle is required. In order to prevent such leakage of the fuel, the disclosed system provides a valve unit in a signal conduit connecting between the control pressure chamber of the pressure regulating valve and a boost pressure introducing port provided at the intake manifold. The valve unit is closed when the engine is stopped. This causes the boost pressure introduced into the control pressure chamber of the pressure regulating valve up to the engine stop to be trapped therein so that the trapped boost pressure is held at least for a certain time after the engine stop.
As described above, the disclosed system relates to the low pressure fuel injection system where the fuel pressure is about 0.2 MPa to 0.3 MPa. This makes it possible to control the fuel pressure using the boost pressure as one of the control factors. On the other hand, in a high pressure fuel injection system for an internal combustion engine of a type where fuel, such as, gasoline is pressurized to near 10 MPa and injected directly into combustion chambers of engine cylinders, a boost pressure is not used for controlling a pressure of the fuel. Accordingly, in the high pressure fuel injection system, the foregoing conventional technique can not be used to prevent leakage of the fuel from the fuel injection valves when the engine is stopped.
In the high pressure fuel injection system, when the engine is stopped, the high pressure fuel held in a fuel piping is heated to a high temperature due to remaining heat of the engine so as to be expanded. As a result, the fuel is likely to leak out through injection holes of the fuel injection valves into the engine cylinders. The leaked fuel may be converted to carbon and adhere to walls of the combustion chambers in the form of carbon deposit. Further, the leaked fuel may increase an amount of contaminants or harmful components in the exhaust gas when the engine is restarted.